Myotonic Dystrophy type 1 (DM1) is autosomal dominant and characterized by progressive weakness, muscle wasting, myotonia, and multisystem impairment (abnormal cardiac conduction, neuropsychiatric impairment, cataracts). Disability occurs at an early stage due to preferential involvement of hand muscles by myotonia and weakness. Death from respiratory failure, aspiration, or cardiac arrhythmia occurs at a median age of 55, usually after several decades of severe disability. Presently there is no treatment other than supportive care. DM1 is caused by expansion of a CTG repeat in the 3′ untranslated of DMPK, the gene encoding dystrophia myotonica protein kinase. Individuals with small CTG repeat expansions of 50-100 repeats generally have mild, late-onset symptoms, whereas large expansions of a thousand or more repeats are associated with severe disease in infancy. Associations between repeat length and disease severity have been made with DNA isolated from circulating blood cells. However, it was found that in many tissues, including skeletal muscle and brain, somatic instability of the expanded CTG repeat leads to much larger expansions, of 1,000 to 5,000 repeats, even in individuals with relatively short expansions in circulating blood cells (Thornton C A, et al. Ann Neurol 1994; 35:104-107). Myotonic dystrophy type 2 (DM2) is similar to DM1, but less common and less severe. DM2 is caused by expansion of a CCTG repeat in intron 1 of the ZNF9 gene, encoding a nucleic acid binding protein.